The present invention relates to a composite trial prosthesis. In particular, embodiments of the present invention relate to a composite trial tibial insert for coupling between a trial tibial tray and a trial femoral component during a knee replacement procedure. The present invention also relates to methods of using composite trial prostheses.
During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. The joint replacement procedure, or joint arthroplasty, may involve the use of a prosthesis which is implanted into one or more of the patient's bones. For instance, during a knee replacement procedure a femoral prosthesis is implanted at the distal end of the femur and a tibial tray prosthesis is implanted at the proximal end of the tibia. Coupled between the two is a tibial insert to adjust the gap between the femoral and tibial prostheses to maintain correct soft tissue tension surrounding the knee. The tibial inserts are available in a range of thicknesses to accommodate different joint spaces between the femur and the tibia and to provide different bearing surfaces to correspond to the selected size and shape of the femoral prosthesis.
A surgical instrument set for performing a knee replacement procedure is marketed by DePuy Orthopaedics, Inc. under the trade mark Sigma High Performance Instruments. The initial surgical steps comprise resecting and shaping the distal end of the femur and the proximal end of the tibia to receive the femoral and tibial prostheses. Trial femoral and tibial prostheses are then temporarily implanted and a trial tibial insert that matches the chosen femoral style and size is inserted and coupled to the trial tibial tray. The joint is then reduced and the knee extended and flexed to assess the stability and alignment of the trial prostheses. It there is instability in the joint then the trial tibial insert is removed and replaced with a trial tibial insert with the same shape bearing, but increased thickness. It will be appreciated that this necessarily requires the Sigma HP instrument set to comprise a large number of trial tibial inserts. Specifically, for every size and style of bearing surface there must be a full range of different thicknesses to accommodate every possible combination.
It is known to provide a smaller range of composite trial tibial inserts in which there is a full range of trial tibial insert bearing components for every required size and shape of femoral prosthesis for the minimum required thickness and a range of standard shaped shims to couple to the bearing components to increase the thickness. However, it will be appreciated that careful marking of the bearing components and the shims is required to prevent the composite thickness of the bearing component plus the shim being incorrectly recorded. Specifically, as the bearing components may be used without shims they must be marked with the minimum thickness they represent (together with information regarding the shape and size of the associated femoral prosthesis). Each shim must also indicate its thickness, or the composite thickness when coupled to the bearing component. If both markings are visible then there is a risk that the marking for the thickness without the shim may be incorrectly recorded as the combined thickness. Furthermore, the shims typically comprise relatively thin sheets that are sandwiched between the bearing component and the trial tibial tray. There may be no other position to mark the shim thickness than on the face of the shim. Consequently, it may not be possible to determine the thickness of the shim without disassembling the bearing component, shim and trial tibial tray. It may be particularly difficult to see the shim markings from the anterior of the joint when the knee joint is reduced.